Supersonic commercial aircraft (Concorde, TU-144) and ultra large aircraft (Airbus A380 and Boeing 747).
I find Concorde and the A380 to be fascinating products. Both were great technological advancements but lost huge amounts of money. Unsustainable amounts. Both were propped up by state aerospace programs, and wound up creating the world's largest aerospace company and the only serious competitor to Boeing. What a ride!
Hopefully we will see a resurgence in the next decade. I'm rooting hard for Boom supersonic.